1. Field of the Invention
The invention relates to a motor drive circuit for supplying voltage and electric current for driving a motor, and more specifically a motor drive circuit capable of preferably serving for switching of a motor drive system.
2. Background of the Invention
FIG. 1 is a schematic circuit diagram showing the configuration of a DC motor and a motor driver (motor drive circuit) for supplying electric current to the motor in order to drive it. An output terminal of the motor driver 2 is connected to a terminal of the DC motor 4. When a control unit 6 turns on a field effect transistor (FET) 8 between the output terminal and the power supply Vdd and turns off an FET 10 between the output terminal and ground, electric current flows from Vdd to the ground connected to another terminal of the motor 4 passing through the motor 4 to drive the motor 4. In addition, when the FET 8 is turned off and the FET 10 is turned on in the condition where the motor 4 is in operation, the counter-electromotive force generated by the rotation of the motor 4 is absorbed by the ground in the form of an electric current, enabling the braking of rotation.
The rotating direction of a DC motor can be switched by changing the direction of the electric current. FIG. 2 is a schematic circuit diagram showing the configuration of a motor driver capable of switching the rotating direction. This is an H-bridge configuration in which motor drivers 2a and 2b corresponding to the motor driver 2 shown in FIG. 1 are connected to both ends of the motor 4. In the circuit shown in FIG. 2, the direction of the electric current flowing through the motor 4 can be changed by changing the combination of on and off for FETs 8a and 10a of the motor driver 2a and FETs 8b and 10b of the motor driver 2b. More specifically, the direction of the electric current when the FETs 8a and 10b are turned on is different from that when the FETs 8b and 10a are turned on, thus enabling a change in the rotating direction of the motor 4.
Major motor drive systems using the motor driver 2, 2a and 2b are the saturation drive, the constant voltage drive and the constant current drive. FIGS. 3 through 5 are circuit diagrams showing configurations of the conventional motor driver 2 in each drive system. In these FIGS. 3 through 5, common components are provided with identical symbols to simplify descriptions. Hereafter symbols “QPk” (k=1, 2, 3, . . . ) denote a metal oxide semiconductor field effect transistor (MOSFET) of the P channel type and symbols “QNk” (k=1, 2, 3, . . . ) denote a MOSFET of the N channel type. In either of these circuits, an entry of L (Low) level into the terminal IN-U turns on QP1 between Vdd and the output terminal OUT and, on the other hand, an entry of H (High) level into the IN-U turns off QP1. Moreover, an entry of L level into IN-L turns on QN1 between ground and the output terminal OUT and, conversely, an entry of H level turns off QN1. Each system is described below.
Firstly, FIG. 3 shows a circuit configuration of a motor driver using the saturation drive system. The pairs of QP2 and QN2, QP3 and QN3 and QP4 and QN4 configure inverters 20, 22 and 24 respectively, invert the input level commonly received by each gate and deliver the inverted level from the connecting points between the drains of each transistor configuring each pair. The inverters 20 and 24 are arranged in series. The input level of IN-U is input to the inverter 20, inverted twice and applied from the output terminal of the inverter 24 to the gate of QP1. The input level of In-U is inverted by the inverter 22 and applied to the gate of QN1.
When IN-U and IN-L are at the L and H levels respectively, QP1 and QN1 are turned on and off respectively and the output voltage Vout corresponding to the power supply Vdd connected to the source of QP1 is obtained from the output terminal OUT. On the other hand, when IN-U and IN-L are at the H and L levels respectively, QP1 and QN1 are turned off and on respectively and the output voltage Vout corresponding to the ground connected to the source of QN1 is obtained from the output terminal OUT.
In the saturation drive system, Vout is set corresponding to the power supply Vdd supplied to the motor driver and the ground potential in this manner. For example, the saturation drive system is used for controlling PWM (pulse width modulation).
FIG. 4 shows a circuit configuration using the constant voltage drive system. This circuit is different from that shown in FIG. 3 in the configuration of the control unit for QP1, which is controlled corresponding to the output of an operational amplifier A1. A switch 26 consisting of the pair of QP5 and QN5 is provided between the output terminal of the operational amplifier A1 and the gate of QP1. The turning on and off of the switch 26 is controlled according to each gate voltage of QP5 and QN5. More specifically, the gate of QP5 is connected to IN-U and the gate of QN5 is connected to the output terminal of an inverter 20, so that when IN-U is at the L level, the switch 26 is turned on and, on the contrary, when IN-U is at the H level, the switch 26 is turned off.
Resistors Rf and Rg are arranged between the output terminal OUT and ground in series, and voltage Va1 at the connecting point of Rf and Rg is used as one input signal of the operational amplifier A1. Va1 is expressed as the equation:Va1=Vout·Rg/(Rf+Rg)  (1)where Vout is a voltage of the output terminal OUT.
The description of the configuration of QP1 is now finished. The configuration of QN1 is similar to that of the saturation drive system shown in FIG. 3. In this circuit configuration, when IN-U and IN-L are at the L and H levels respectively, the switch 26 is turned on to turn on and off QP1 and QN1 respectively. At this time, the operational amplifier A1 controls the conduction state of QP1 so that the voltage Va1 input to one input terminal is equal to the reference voltage Vref input to the other input terminal. As a result, the output voltage Vout is set at a constant voltage in proportion to Vref as shown in the following equation:Vout=Vref·(Rf+Rg)/Rg  (2)
FIG. 5 shows a circuit configuration using the constant current drive system. This circuit is different from that shown in FIG. 3 in the configuration of the control unit for QN1, which is controlled corresponding to the output of an operational amplifier A2. A switch 28 consisting of the pair of QP6 and QN6 is provided between the output of the operational amplifier A2 and the gate of QN1. The turning on and off of the switch 28 is controlled according to each gate voltage of QP6 and QN6. More specifically, the gate of QP6 is connected to IN-L and the gate of QN6 is connected to the output terminal of an inverter 22 so that when IN-L is at the L level, the switch 28 is turned on and, on the contrary, when IN-L is at the H level, the switch 28 is turned off.
A resistor Rt is arranged between the source of QN1 and ground in series and the voltage Va2 at the connecting point of QN1 and Rt is input to another input terminal of an operational amplifier A2. Va2 is expressed as the equation:Va2=Ids·Rt  (3)where Ids is an electric current flowing between the drain and the source of QN1.
The description of the configuration of QN1 is now finished. The configuration of QP1 is similar to that of the saturation drive system shown in FIG. 3. In this circuit configuration, when IN-U and IN-L are at the H and L levels respectively, the switch 28 is turned on to turn on and off QN1 and QP1 respectively so that the electric current Iout flowing to the output terminal OUT becomes Ids. At this time, the operational amplifier A2 controls the conduction state of QN1 so that the voltage Va2 input to one input terminal is equal to the reference voltage Vref input to the other input terminal. As a result, the output current Iout of a motor driver using the constant current system is set at a constant current in proportion to Vref as shown in the following equation:Iout=Vref/Rt  (4)
In the circuit shown in FIG. 5, Iout serves to introduce electric current from the terminal OUT. For example, in the configuration shown in FIG. 2 that is adopted using the motor driver shown in FIG. 5, when IN-U and IN-L are at the H and L levels respectively in one motor driver 2a, the motor driver 2a serves to introduce a constant current Iout. At this time, when IN-U and IN-L are made to be at the L and H levels respectively in another motor driver 2b to turn on QP1, and the constant current Iout flows from Vdd of the motor driver 2b to ground of the motor driver 2a through the motor 4 to drive the motor 4.
Among these multiple drive systems, there is a need to drive a motor by switching between the constant voltage drive and the saturation drive or to drive a motor by switching between the constant current drive and the saturation drive. On the other hand, there is a problem in the conventional method in that the circuit size of a motor driver is large because motor drivers required for each drive system are provided as independent circuits.
In the motor driver using the saturation drive system shown in FIG. 3, the output voltage is fixed by Vdd and the ground potential, through being disabled in order to set the constant voltage Vout at the desired value between them. In addition, the desired constant current Iout can also not be obtained. On the other hand, in the motor driver using a constant voltage drive system shown in FIG. 4 and the motor driver using a constant current drive system shown in FIG. 5, the configuration of the variable Vref enables the setting of Vout at a value similar to the output voltage of the saturation drive system. Motor drivers using such constant voltage or constant current drive systems, however, have low control responsiveness because feedback control is applied using an operational amplifier. Thus there is a problem where these motor drivers are not suitable for switching operations at required frequencies, such as in PWM control.